Large tractor trucks typically have air-conditioning systems similar to automobiles. The truck engine drives a compressor that compresses refrigerant and delivers it to a condenser. The condenser converts the hot gaseous refrigerant to a liquid refrigerant. The refrigerant flows to an evaporator where it undergoes a pressure drop, converting the refrigerant to a cold gas. An interior fan flows air through the evaporator into the interior of the vehicle. The condenser is cooled by the main engine fan, which also flows air through the engine radiator.
For heating, a heater coil or element is mounted in the vehicle in communication with the radiator via hoses. A portion of the hot engine coolant flows through the heater coil. The interior fan flows air through the heater coil to heat the interior of the vehicle.
Many large trucks have sleeper compartments attached to the cab for allowing the driver to rest. In most cases, for heating and cooling, the operator continues to operate the main truck engine at idle while sleeping in order to run the air-conditioner or heater. Often, the sleeper compartment will have a sleeper compartment evaporator, heater coil and blower. The evaporator in the sleeper compartment is in parallel with the truck cab evaporator, and the heater in the sleeper compartment is in parallel with the truck cab heater. The main air conditioner compressor and condenser supply refrigerant to the sleeper compartment evaporator, consequently, the truck engine must be operated. While idling, the main engine generates far more power than needed for heating and cooling, consequently consumes considerable fuel while the driver is resting.
Generators have been mounted to large trucks for generating 115 volt AC power. An auxiliary engine, normally diesel, is located in a compartment along with a generator. Usually, a separate 110 volt air-conditioning unit mounts to the sleeper compartment or cab. The air-conditioning unit has an electrical motor that drives the compressor and the fan. For heat, an electrical resistance element may be employed, or the air-conditioner may be operated as a heat pump. Heating by a 110 volt air conditioner unit, however, consumes a considerable amount of power.
Another approach for heating and cooling while the truck is stopped is to utilize a 110 volt air conditioning unit and a power cord that extends to a power service receptacle at a rest stop. Many rest stops, however, do not have such provisions for connecting a vehicle to electrical power.
Also, auxiliary power units are known that use an auxiliary engine to directly drive an auxiliary air conditioner compressor. An auxiliary condenser is mounted in the auxiliary housing. An auxiliary evaporator, heating element, and blower are mounted in the sleeper compartment, typically under the bunk or bed. The auxiliary blower discharge is independent of the ducts of the rear sleeper compartment air conditioning unit. The auxiliary engine normally also drives an alternator for supply DC power to the blower and other DC equipment.